Solvent extraction of highly aromatic charge stocks



g- 6, 1958 D. B. BROUGHTON 3,396,101

SOLVENT EXTRACTION OF HIGHLY AROMATIC CHARGE STOCKS Filed Aug. 3]., 1966 //V l/E/V TOR: Donald B. Brough/on A TTOR/VEYS United States Patent M 3,396,101 SOLVENT EXTRACTION 0F HIGHLY AROMATIC CHARGE STOCKS Donald B. Broughton, Evanston, Ill., assignor to Universal Oil Products Company, Des Plaines, 111., a corporation of Delaware Filed Aug. 31, 1966, Ser. No. 576,343 20 Claims. (Cl. 208313) ABSTRACT OF THE DISCLOSURE Process for recovering aromatic hydrocarbons from a charge stock containing at least 75 wt. percent aromatics which comprises passing the charge stock in admixture with lean solvent to an extractive stripper column as a single liquid feed. A non-aromatic overhead fraction containing some aromatics is withdrawn therefrom and passed to an extraction zone for aromatic recovery. A resulting rich solvent is passed from the extraction zone to the extractive stripper column as a second feed stream thereto. In a particular embodiment, the single feed stream comprising lean solvent and hydrocarbon charge stock enters the top of the extractive stripping column while the rich solvent enters at a locus below. In a preferred embodiment, the rich solvent, the lean solvent, and the hydrocarbon charge stock are passed in admixture as a single feed to the extractive stripper. A bottoms fraction is Withdrawn from the extractive stripper and separated to provide a high purity aromatic product and a lean solvent which is recycled to the extractive stripper column and to the extraction zone.

The present invention relates to the solvent extraction of aromatic hydrocarbons from a mixture of hydrocarbon components. More particularly, the present invention relates to the solvent extraction of aromatic hydrocarbons from a highly aromatic hydrocarbon feed. Most specifically, the present invention relates to the recovery of aromatic hydrocarbons from a highly aromatic feedstock in a process comprising solvent extraction in combination with extractive stripping.

As used herein, the term solvent extraction refers to countercurrent liquid-liquid extraction, while the term extractive stripping refers to the stripping of non-aromatic rafiinate vapor from a rich solvent in a process wherein the aromatic extract substantially remains dissolved in liquid solvent.

Solvent extraction of aromatic hydrocarbons from hydrocarbon mixtures is well known in the art of hydrocarbon processing, and a typical solvent which may be utilized within the practice of the present invention comprises a solvent of the sulfolane type. The solvent possesses a five membered ring containing one atom of sulfur and four atoms of carbon, with two oxygen atoms bonded to the sulfur atom of the ring. Generally, the sulfolane type solvents may be indicated as having the following structural formula:

wherein R R R and R are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an alkoxy radical having from one to eight carbon atoms, and an arylalkyl radical from one to twelve carbon atoms. Other solvents which may be included within this process are 3,396,101 Patented Aug. 6, 1968 the sulfolenes such as 2-sulfolene or 3-sulfolene which have the following structures:

S S (I311 (EH2 $112 (IJHZ JHOH CH CH 2-sulf0lene 3-sulfolene Other typical solvents which have a high selectivity for separating aromatics from non-aromatic hydrocarbons and which may be processed within the scope of the present invention are 2-methylsulfolane, 2,4-dimethylsulfolane, methyl 2-sulfonyl ether, n-aryl-3-sulfonyl amine, 2 sulfonyl acetate, diethylene glycol, various polyethylene glycols, dipropylene glycol, various polypropylene glycols, 'dimethyl sulfoxide, N-methyl pyrrolidone, etc.

The specifically preferred solvent chemical which is processed within the scope of the present invention is sulfolane, having the following structural formula:

It is known in the art of aromatics extraction that when the aromatic content of the hydrocarbon charge stock becomes very high, the charge stock and the solvent may become completely miscible. Thus, in sulfolane-type solvent extraction and in glycol-type solvent extraction systems, when the aromatic content of the charge stock exceeds about wt. percent, the distinct hydrocarbon phase and the distinct solvent phase disappear and a single phase of liquid results. The aromatic concentration above 75 wt. percent at which this phenomenon occurs will vary depending upon the temperature of the system, the type of solvent utilized, the boiling range of the hydrocarbon charge stock, type or molecular weight of the aromatics, etc. Since solvent extraction processing comprises the countercurrent contacting of two distinct liquid phases in an extraction vessel, it is apparent that high aromatic content charge stocks cannot be readily processed by direct introduction of the hydrocarbon charge stock into the extraction vessel.

It is therefore an object of the present invention to recover high purity aromatic hydrocarbons from a hydrocarbon stock containing an aromatics concentration of 75 wt. percent or more. It is a more particular object of the present invention to recover high purity aromatic hydrocarbons in -a solvent extraction process wherein the hydrocarbon charge stock contains at least 75 wt. percent aromatics. It is a further object of the present invention to recover high purity aromatics from the highly aromatic hydrocarbon stock in a solvent extraction process wherein a very high recovery of aromatics is obtained in a facile manner.

Therefore, in accordance with the practice of the present invention, a broad embodiment comprises the recovery of aromatic hydrocarbons from a hydrocarbon charge stock containing at least 75 Wt. percent aromatics by introducing said charge stock with a first portion of a lean solvent stream hereinafter specified and with a first rich solvent stream hereinafter specified into an extractive stripping means as a single miscible feed stream, removing from the extractive stripping means a nonaromatic stream containing some aromatics, introducing at least a part of said non-aromatic stream into an extraction means wherein said stream is extracted by a second portion of said lean solvent stream hereinafter specified, withdrawing from the extraction means a rafiinate stream having substantial freedom from aromatic hydrocarbons, withdrawing from the extraction means said specified first rich solvent stream, passing a second rich solvent stream from the extractive stripping means to a distillation means, recovering a substantially pure aromatic product stream from the distillation means, and withdrawing from the distillation means said lean solvent stream specified.

Another embodiment of the present invention comprises processing a hydrocarbon charge stock containing at least 75 wt. percent aromatics by introducing said charge stock with a first portion of a lean solvent stream hereinafter specified into an extractive stripping means as a single miscible feed stream, introducing a first rich solvent stream hereinafter specified into the extractive stripping means, removing from the extractive stripping means a non-aromatic stream containing some aromatics, introducing at least a part of said non-aromatic stream into an extraction means wherein said stream is extracted by a second portion of said lean solvent stream hereinafter specified, withdrawing from the extraction means a raffinate stream having substantial freedom from aromatic hydrocarbons, withdrawing from the extraction means said specified first rich solvent stream, passing a second rich solvent stream from the extractive stripping means to a distillation means, recovering a substantially pure aromatic product stream from the distillation means, and withdrawing from the distillation means said lean solvent stream specified.

In a further embodiment of the present invention, the recovery of aromatic hydrocarbons from a hydrocarbon charge stock containing at least 75 wt. percent aromatics is accomplished by introducing said charge stock with a first portion of a lean solvent stream hereinafter specified into the top of an extractive stripping means as a single miscible feed stream, introducing a first rich solvent stream hereinafter specified into the extractive stripping means at a point below the top, removing from the extractive stripping means a non-aromatic stream containing aromatics, introducing at least a part of said nonaromatic stream into an extraction means wherein said stream is extracted by a second portion of said lean solvent stream hereinafter specified, withdrawing from the extraction means a raffinate stream having substantial freedom from aromatic hydrocarbons, withdrawing from the extraction means said specified first rich solvent stream, passing a second rich solvent stream from the extractive stripping means to a distillation means, recovering a substantially pure aromatic product stream from the distillation means, and withdrawing from the distillation means said lean solvent stream specified.

These embodiments are set forth in the accompanying FIGURE I which consists of a simplified flow diagram of the invention process.

In the art of aromatics extraction, the typical solvent composition comprises a mixture of water and one or more of the solvent chemicals previously noted. The particularly preferred solvent composition of the present invention comprises water and sulfolane. In extracting aromatic hydrocarbons from a hydrocarbon mixture, it is known that parafiins are the least soluble followed in increasing order of solubility by naphthenes, olefins, diolefins, acetylenes, sulfur-containing hydrocarbons, nitrogen-containing hydrocarbons, oxygen-containing hydrocarbons, and aromatic hydrocarbons. It is the practice to regulate the solubility of the hydrocarbons within the solvent composition by varying the water content thereof. Thus, by adding more water to the solvent, the solubility of all components in the hydrocarbon mixture is decreased, but the solubility difference between components (selectivity) is increased. The net effect is to decrease the number of contacting stages required to achieve a given purity of aromatic extract, or to increase the resulting purity of the aromatic extract when the number of contacting stages is held constant.

The presence of Water in the solvent composition provides a further processing benefit in that it introduces a relatively volatile material into the extractive stripping means. The water of the solvent composition is vaporized therein and provides assistance in stripping all traces of non-aromatic hydrocarbon out of the aromatic-rich solvent.

It is therefore the practice to provide that the solvent composition contain from about 0.1% to about 20% by weight of water. When the solvent system comprises chemical sulfolane, it is preferable that the solvent composition contain from about 0.1% to about 1.0% water, while a solvent comprising a polyalkylene glycol preferably contains from about 6% to 12% Water.

The hydrocarbon charge stocks which are usually processed within the scope of the solvent extraction art are depentanized gasoline boiling range or naphtha boiling range stocks. These stocks will be comprised of hydrocarbons having from about six to twenty carbon atoms per molecule, but normally, aromatics extraction is practiced upon stocks containing only from six to twelve carbon atoms per molecule (C to C Since benzene, toluene, and xylenes are the commercially preferred aromatic hydrocarbons, it is desirable to fractionate the gasoline or naphtha to provide a concentrate of aromatics in the C to C range which may then be more effectively solvent-extracted to recover the aromatics contained therein. A preferred source of such an aromatics concentrate is gasoline from a catalytic reforming process. Other sources of such high aromatic concentrates are pyrolysis process by-product liquids and coke oven by-product liquids. The pyrolysis bysproduct liquids and coke oven liquids are not only rich in aromatics, but are normally high in olefins and diolefins, and it is therefore necessary to hydrotreat these stocks since there unsaturated hydro carbons reduce the effectiveness of the solvent extraction process.

When the concentration of aromatics in the solvent extractor charge stock reaches wt. percent or more, and the solvent composition and extractor operating conditions are sufficient to prevent the maintenance of distinct solvent phase and liquid phase within the extractor, it is known in the art that the condition of complete miscibility may be reversed by recycling a part of the nonaromatic rafiinate to the charge stock thereby reducing the concentration of aromatics. While such a practice is effective in enabling high aromatic concentrates to be processed by solvent extraction, it has many inherent detrimental features. Because of the increase in hydrocarbon rate to the extractor due to the recycle of raffinate, the solvent circulation rate must be increased. The net effect is that increased size is required for the extractor vessel, the rafiinate recovery facilities, and the solvent fractionation facilities. The summary disadvantages of this processing scheme is, therefore, that it requires a greater capital expense and a greater operating expense for a given recovery of aromatic hydrocarbon product.

A further processing technique which is employed in high aromatic concentrates is extractive distillation. In an extractive distillation process the high aromatic hydrocarbon is charged to about the center of the column, the lean solvent is introduced above the hydrocarbon feed point but below the top of the column, and a non-aromatic reflux is introduced into the top of the column. The extractive distillation column thereby contains a stripping zone below the hydrocarbon feed point, an aromatics absorption zone between the feed point and the lean solvent inlet, and a recifying zone above the lean solvent inlet. The lean solvent contacts the hydrocarbon vapors in the absorption zone and selectively dissolves the more soluble components comprising aromatic hydrocarbons. The resulting rich solvent is then stripped of its non-aromatic content in the stripping zone by vapors generated in the column reboiler. A raf-- finate vapor leaves the top of the column and is can densed. A part of the non-aromatic ratfinate is then refluxed at the top of the column in order to reduce the loss of solvent and aromatics overhead. A final rich solvent which is substantially free of nonaromatic leaves the bottom of the extractive distillation column and is sent to a subsequent distillation column for recovery of high purity aromatic product and recovery of a lean solvent composition which is recycled to the extractive distillation column.

One of the disadvantages of the extractive distillation process is that it inherently has a high pressure drop due to the number of decks required in the column and due to the high solvent and reflux rates which are required to achieve a satisfactory recovery of aromatics at a high purity. Under certain operating conditions, and particularly when seeking a recover C and C or heavier aromatics, the boiling point elevation imposed on the system due to the pressure drop may be suflicient to cause thermal cracking of the solvent chemical. When the solvent comprises chemical sulfolane, excessive decomposition is experienced when the temperature exceeds 360 F. When the solvent composition comprises diethylene glycol, dipropylene glycol, or triethylene glycol, excessive decomposition of the glycol is encountered when the temperature exceeds 380 F.

A further disadvantage of the extractive distillation process is that the lean solvent is not being utilized to take advantage of its known solvency and selectivity characteristics. The lean solvent entering the absorption zone must contact vaporized aromatic hydrocarbons in order to be enriched with aromatics. The solvency of the lean solvent is most effective in a liquid-liquid contact, however, and the scrubbing of hydrocarbon vapor by the lean solvent requires more decks to reach a given aromatic content than what would be required in a liquid-liquid extractor.

In addition, the refluxing of non-aromatic raflinate will normally cause a three phase system to exist at the top of the column. This system consisting of a vapor phase, a hydrocarbon liquid phase, and a, solvent liquid phase will exist because the lean solvent does not have sutficient solvency for the non-aromatic reflux. In order to overcome the problems inherent in such a three phase system, the lean solvent must be introduced at an exces sively high rate in order to dissolve the non-aromatic hydrocarbons.

These disadvantages and other adverse effects which are experienced in the prior art are readily overcome by the embodiments of the present invention. An understanding of these embodiments and the effectiveness of the inventive process may be obtained by now referring to Figure I.

In a first embodiment, a highly aromatic charge stock containing at least 75 wt. percent aromatics enters the inventive process via line 1. A first rich solvent stream containing aromatic hydrocarbons enters line 1 by means of line 13 from a source to be specified hereinbelow, and a first portion of a lean solvent stream to be specified hereinbelow enters line 1 by means of line 28. The combined stream of hydrocarbon charge stock, rich solvent, and lean solvent passes through line 1 and into mixer means 2 which may comprise any suitable mixing apparatus such as an in-line mixer containing a motor driven propeller or such as a series of mixing orifices. Mixer means 2 is operated under conditions suflicient to thoroughly mix the input from line 1 and to discharge a completely miscible single phase liquid into line 3. In general, these conditions comprise a suflicient amount of lean solvent to dissolve the hydrocarbon, an elevated temperature, and a pressure sufliciently great to eliminate the possibility of vaporization within the mixer 2. When the solvent comprises chemical sulfolane, the ratio of the volume of lean solvent entering line 1 from line 28 to the volume of hydrocarbon charge stock entering line 1 may range from about 30:10 to 60:10 but will normally be about 4.0:1.0. When the solvent comprises at least one glycol from the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol, the volumetric ratio may range from about 4.5: 1.0 to 8.0: 1.0 but will normally be about 7.0. When the solvent composition comprises chemical sulfolane and water, the temperature may range from 150 F. to 300 F. but will normally range from 180 F. to 220 F. When the solvent composition comprises at least one of the specified glycols and water, the temperature may range from 200 F. to 400 F. but will normally range from 275 F. to 320 F. It must be noted that the solvent to hydrocarbon ratio and the temperature will depend upon the boiling point of the hydrocarbon and the component distribution of the molecular species or aromatics contained therein. In all events, mixer 2 will normally be maintained at a pressure in the range of from 50 p.s.i.g. to 150 p.s.i.g.

The resulting single phase liquid stream leaves mixer 2 via line 3 and passes through control valve 3' and is partially vaporized. Control valve 3' comprises a backpressure valve which holds the elevated pressure at mixer 2 in order to assure that all aromatic hydrocarbons are dissolved in the solvent composition. The partially vaporized stream continues via line 3 and enters the top of extractive stripper column 4. Extractive stripper column 4 is operated under conditions suflicient to strip substantially all non-aromatic hydrocarbons from the solvent entering via line 3. The pressure within the column is maintained sulficiently low to provide that the reboiler temperature of the extractive stripper 4 does not reach the point at which excessive thermal decomposition of the solvent chemical will occur. When the solvent cornprises sulfolane, the column temperature must be maintained below 360 F. and when the solvent comprises at least one of the specified glycols, the temperature must. not exceed 380 F. In order to maintain these tempera-- ture limitations it is, therefore, the practice to operate extractive stripper 4- at from 1 p.s.i.g. to about 20 p.s.i.g. when the hydrocarbon contains C -C aromatics, and to operate extractive stripper 4 at subatmospheric pressure when the aromatics consist primarily of C and heavier aromatics.

A non-aromatic vapor leaves the top of extractive stripper 4 via line 5. This vapor stream contains substantially all non-aromatics which entered column 4 via line 3, some low-boiling aromatics, some solvent chemical, and some water. The vapor enters condenser 6 and is cooled therein to F. or less. The resulting liquid enters separator 8 by means of line 7 wherein an aqueous phase is separated from the hydrocarbon. The aqueous phase contains substantially all of the solvent which was stripped overhead from extractive stripper 4, and a stream of this aqueous phase is withdrawn from separator 8 via line 9 and normally sent to a solvent recovery system not shown. A non-aromatic hydrocarbon liquid containing some aromatics leaves separator 8 and passes to an extraction means 11 via line 10. Extraction means 11 may comprise any suitable liquid-liquid extraction device as a packed tower, a trayed tower, or a rotating disc contactor (RDC column). The non-aromatic hydrocarbon liquid is extracted therein by a second portion of the lean solvent stream, to be specified hereinbelow, which enters extractor 11 by means of line 14.

Extractor 11 is maintained at conditions suflicient to provide that substantially all of the aromatic hydrocarbons are removed from the non-aromatic hydrocarbon liquid, and that substantially all of the non-aromatic hydrocarbons which entered the inventive process via line 1 are withdrawn from the extractor 11 via line 12 as a non-aromatic raffinate.

The extractor is maintained at a pressure sufficiently elevated to provide that no vaporization of the hydrocarbon will occur. Although the pressure which is required will depend upon the boiling point of the hydrocarbon being extracted, extractor 11 is normally maintained at a pressure from about 50 p.s.i.g. to p.s.i.g. In order to extract substantially all of the aromatics entering the extractor via line 10 when the lean solvent comprises chemical sulfolane and water, the temperature of extraction is kept in the range of from about 150 F. to 300 F. and normally in the range of from about 180 F. to 220 F. When the solvent composition comprises water and at least one of the specified glycols, the temperature is maintained in the range of from 200 F. to 400 F. and normally in the range of from 275 F. to 320 F. In addition, the volumetric ratio of lean solvent entering via line 14 to the non-aromatic hydrocarbon entering via line 10 may be in the range of 1.0: 1.0 to 2.5: 1.0 and will normally be about 1.5 :1.0 when the solvent comprises sulfolane, while the ratio may be in the range of about 5.0:1.0 to 80:1.0 and normally in the range of about 5.5:l.0 to 7.0210 when the solvent comprises at least one of the glycols specified. It must be noted that the specific operating conditions within extractor 11 will depend upon the aromatic content of the non-aromatic hydrocarbon entering via line 10 and the type and distribution of the molecular species of aromatics contained therein. In addition to the non-aromatic rafiinate which is withdrawn from extractor -11 via line 12, a rich solvent stream containing substantially all aromatics which enter the extractor and containing some light non-aromatics leaves via line 13. This rich solvent stream passes through blockvalve 13' and continues via line 13 until it enters line 1 as the first rich solvent stream specified hereinabove.

A second rich solvent stream leaves the bottom of extractive stripper column 4 via line 15. A part of this rich solvent stream is Withdrawn by means of line 16 and passed through reboiler means 17 wherein the temperature is elevated and a part of the aromatic hydrocarbons are vaporized. The heated stream leaves reboiler 17 via line 18 and returns to extractive stripping column 4 where the vaporized hydrocarbon serves as a stripping medium in order to remove non-aromatic hydrocarbons from the rich solvent passing downflow in column 4. The remaining portion of the second rich solvent stream continues by means of line to control valve 15. The pressure is reduced through control valve 15', and a part of the hydrocarbon in the stream is vaporized. The total stream then continues by means of line 15 to distillation column 19.

Distillation column 19 serves to recover the aromatic hydrocarbons from the rich solvent and to provide a lean solvent for return and reuse in the inventive process. The pressure at which column 19 operates is maintained sufiiciently low to provide that the temperature of distillation does not exceed the limits which are required in order to minimize thermal decomposition of the solvent chemical. Normally, this pressure will range from 100 to 400 millimeters of mercury. The temperature limit at the reboiler of column 19 is 360 F. when the solvent comprises sulfolane, and 380 F. when the solvent comprises a specified glycol. In order to further maintain the temperature below these preferred limits, stripping steam is also provided at a rate sufficient to insure removal of substantially all hydrocarbons from the solvent. The stripped solvent passes from the bottom of column 19 by means of line 28, and a part of this solvent is withdrawn by line 29. Stripping steam is introduced by line into line 29, and the combined stream enters reboiler means 31 wherein the total stream is further heated and passed into column 19 by means of line 32. The stripping steam passes up column 19 and strips substantially all of the hydrocarbons out of the solvent entering the column.

The aromatic hydrocarbon vapor which is removed is withdrawn from column 19 with the stripping steam by means of line 20 and passed to condenser 21 wherein the vapor is condensed and cooled to a temperature of 100 F. or less. The condensed liquid then passes to separator 23 by means of line 22. Subatmospheric pressure is maintained in column 19 and in separator 23 by withdrawing all non-condensed gas and vapor via line 24, while the stripping steam condensate is separated and withdrawn from separator 23 by means of line 25. Since this steam condensate may contain dissolved solvent which has been stripped overhead, this aqueous stream is normally sent to a solvent recovery system not shown. The hydrocarbon phase which is separated is withdrawn by means of line 26, and a part is returned to column 19 as reflux. A second part of this aromatic hydrocarbon is withdrawn from line 26 and leaves the inventive process by means of line 27 as the desired high purity aromatic product.

A second part of the stripped solvent in line 28 leaves column 19 as a final lean solvent for recirculation. The lean solvent stream flows via line 28 and is finally divided into two portions. One portion of the lean solvent is withdrawn from line 28 and passed by means of line 14 to extracto -r 11 as the previously specified second portion of the lean solvent stream. The remaining portion of the lean solvent continues in line 28 and is introduced into line 1 as the first portion of the lean solvent stream specified hereinabove.

In a second embodiment of the inventive process, the fiow scheme as now set forth, is identical with the exception that the first rich solvent stream which leaves extractor 11 is not introduced into line 1. The first rich solvent stream leaves extractor 11 by means of line 13 and is withdrawn .from line 13 by way of line 33. In this embodiment, block valve 13 and block valve 34 are closed, and block valve 33 is opened. The first rich solvent stream flows in line 33 through valve 33 and llIli.O the top of the extractive stripping column 4. The hydrocarbon charge stock which is introduced in line 1 is once again dissolved in the lean solvent which is introduced into line 1 by means of line 28. The resulting nich solvent leaving mixer 2 is then introduced into the top extractive stripper 4 by means of line 3 as previously set forth in the first embodiment.

In a further embodiment of the inventive process, the first rich solvent stream leaving extractor 11 is not introduced into the top of the extractive stripping column 4. In this embodiment, block valves 13' and 33 are closed, and block valve 34 is open. The first rich solvent stream leaves extractor 11 through line 13 and line 33 and is then passed through line 34 and block valve 34 into the extractive stripping column 4 below the top of the column. In this manner the first rich solvent stream which normally contains some light non-aromatic hydrocarbons is introduced in a location whereby the light nonaromatic hydrocarbons will vaporize and pass through the upper regions of column 4 thereby providing a light hydrocarbon stripping medium. The light non-aromatic hydrocarbons are preferentially soluble in the solvent and will displace heavier non-aromatics from the solution. The heavier nonaromatics thereby pass into the vapor phase and are removed overhead via line 5. The lower-boiling nonaromatics are then easily stripped from the solvent in the lower regions of extractive stripper 4 where the higher column temperatures are encountered.

Modifications of the present invention will be readily perceived by those skilled in the art. For example, it is possible to remove the rich solvent from extractor 11 via line 13 and not send it to extractive stripper 4, but to recover aromatics from this rich solvent in external distillation means. In addition, it is possible to bring fresh lean solvent from an external source into line 14 for processing in extractor 11, and thereby replace in part or entirely the lean solvent leaving line 28 via line 14 as set forth in the preferred embodiments. Similarly, fresh lean solvent could be injected into line 1 and thereby replace at least a part of the lean solvent which enters line 1 via line 28. Such modifications and others which are readily ascertainable are not preferred, however, since they would require additional equipment and operating expense.

It may be seen that the disadvantages and adverse effects which are experienced in the prior art are readily overcome by the embodiments of the present invention. The present invention does not seek to avoid the problem of complete miscibility of high aromatic stocks in the solvent composition, but rather seeks to take advantage of it. The hydrocarbon and solvent are thoroughly mixed as liquids thereby eliminating the inefiicient liquid-vapor contacting of solvent and aromatics in the absorption zone of the prior art extractive distillation column. The extractive stripping column 4 then removes substantially all of the non-aromatics from the solvent composition as the stripping column overhead. The recovery of aromatics contained in the stripping column overhead is then accomplished in extractor 11, thereby eliminating the inefficient rectifying zone of the prior art extractive distillation column. Since extractor 11 does not process the entire hydrocarbon charge stock, but only the substantially non-aromatic portion, extractor 11 is reduced in size, and its capital and operating expenses are reduced.

In addition to overcoming the disadvantages of the prior art, it may be seen that the preferred embodiments of the present invention have specific advantages of operational flexibility. Since the highly aromatic charge stock is completely dissolved in the solvent as the first step of the inventive process, column 4 is not required to provide a means of absorbing aromatics in the solvent, but only a means of removing non-aromatics from the solvent. Column design and column operation are simplified since no attempt is made to maximize aromatic recovery in extractive stripper 4. The inherent loss of aromatics and solvent in the non-aromatic vapor leaving extractive stripper 4 is tolerated since the solvent is easily recovered from the aqueous stream withdrawn via line 9, while the aromatic hydrocarbons are efliciently recovered in extractor 11. The design and operation of extractor 11 is also simplified since the extractor is not required to recover a pure aromatic stream, but only to provide that the non-aromatic raffinate leaving in line 12 contains a inimimum amount of aromatic hydrocarbons. The aromaticrich solvent leaving extractor 11 may contain a considerable amount of non-aromatics without causing a reduced purity of the final aromatic product, since the nonaromatics are readily stripped out of the solvent in the extractive stripper 4. By proper adjustment of operating conditions within the extractor 11 and the extractive stripper 4, the present invention thereby provides a novel method for optimizing the recovery and the purity of aromatics from a highly aromatic hydrocarbon charge. Those knowledgeable in the art will readily ascertain the specific operating conditions required for any specific charge stock and solvent composition in order to so optimize aromatic recovery and purity in the inventive process.

It may also be noted that the use of two separation columns instead of only one column for the removal of hydrocarbons from the solvent also introduces the advantage of operational flexibility. By use of extractive stripper 4 in the removal of non-aromatic hydrocarbons from the solvent and use of distillation column 19 for removal of aromatic hydrocarbons from the solvent, optimum advantage may be taken of the volatilty characteristics of the different hydrocarbon species. Extractive stripper 4 will normally operate at a substantially higher pressure than distillation column 19, with stripper 4 normally operating at from 1 p.s.ig. to 2-0 p.s.i.g., and column 19 normally operating at subatmospher-ic pressure. In addition, the two column system allows the aromatic extract to be refluxed without danger of contamination by non-aromatic hydrocarbons.

The invention claimed:

1. Process for recovering aromatic hydrocarbons from a charge stock containing at least 75 wt. percent aromatics which comprises (a) passing said charge stock in admixture with lean solvent and in admixture with a rich solvent stream hereinafter specified into an extractive stripping means as a single feed stream;

(b) separating said single feed stream into an over-' head fraction containing nonaromatic hydrocarbons and into a bottom fraction containing aromatic hydrocarbons;

(c) introducing at least a part of said overhead fraction into an extraction means wherein said fraction is contacted by lean solvent stream under conditions sufficient to remove aromatic hydrocarbons;

(d) withdrawing from the extraction means a raifinate stream having substantial freedom from aromatic hydrocarbons;

(e) withdrawing from the extraction means a rich solvent stream containing aromatic hydrocarbons as said rich solvent stream specified in Step (a); and

"(f) separating said bottom fraction into a product stream containing aromatics and into a lean solvent stream having substantial freedom from aromatic hydrocarbons.

2. Process of claim 1 wherein said lean solvent of Step (a) comprises at least aportion of said lean solvent stream of Step (f).

3. Process of claim 1 wherein said lean solvent of Step (c) comprises at least a portion of said lean solvent stream of Step (f).

4. Process of claim 1 wherein said lean solvent of Step (a) com-prises a first portion of said lean solvent stream of Step (f), and said lean solvent of Step (c) comprises a second portion of said lean solvent stream of Step (f).

'5. Process of claim 1 wherein said lean solvent of Step (a) and of Step (c) comprises a sulfolane-type chemical of the general formula:

wherein R R R and R are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an arylalkyl radical having from one to twelve carbon atoms, and an alkoxy radical having from one to eight carbon atoms.

6. Process of claim 5 wherein said lean solvent of Step (a) and of Step (c) comprises sulfolane.

7. Process of claim 1 wherein said lean solvent of Step (a) and of Step (c) comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.

8. Process of claim 1 wherein said lean solvent of Step (a) and of Step (c) comprises at least one polyalkylene glycol.

9. Process of claim 8 wherein said lean solvent of Step (a) and of Step (0) comprises at least one of the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.

10. 'Process for recovering aromatic hydrocarbons from a charge stock containing at least 75 Wt. percent aromatics which comprises (a) passing said charge stock in admixture with lean solvent into an extractive stripping means;

(b) passing a rich solvent stream hereinafter specified into said extractive stripping means;

(c) maintaining said extractive stripping means under conditions sufficient to provide an overhead fraction containing nonaromatic hydrocarbons and a bottom fraction containing aromatic hydrocarbons;

(d) introducing at least a part of said overhead fraction into an extraction means wherein said fraction is contacted by lean solvent under conditions sufficient to remove aromatic hydrocarbons;

(e) withdrawing from the extraction means a raflinate stream having substantial freedom from aromatic hydrocarbons;

(f) withdrawing from the extraction means a rich solvent stream containing aromatic hydrocarbons as said rich solvent stream specified; and

'(g) separating said bottoms fraction into a product stream comprising aromatic hydrocarbons and into 11 a lean solvent stream having substantial freedom from aromatic hydrocarbons.

11. Process of claim 10 wherein said charge stock is passed in admixture with said lean solvent of Step (a) into the top of the extractive stripping means, and said rich solvent stream is passed into the extractive stripping means at a locus below.

12. Process of claim 10 wherein said lean solvent of Step (a) comprises at least a portion of said lean solvent stream of Step (g).

13. Process of claim 10 wherein said lean solvent of Step (d) comprises at least a portion of said lean solvent stream of Step (g).

14. Process of claim 10 wherein said lean solvent of Step (21) comprises a first portion of said lean solvent stream of Step (g), and said lean solvent of Step ((1) comprises a second portion of said lean solvent stream of Step (g).

15. Process of claim 10 wherein said lean solvent of Step (a) and of Step (d) comprises a sulfolane-type 4 chemical of the general formula:

CH-Rr H-Rs Rr-CH fir-CH- wherein R R R and R are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an arylalkyl I 19. Process of claim 18 wherein said lean solvent of Step (a) and of Step (d) comprises at least one of the groups consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.

20. Process for recovering aromatic hydrocarbons from a charge stock containing at least 75 wt. percent aromatics which comprises:

(a) passing said charge stock in admixture with lean solvent into an extractive stripping means as a single feed stream;

(b) separating said single feed stream into an overhead fraction containing nonaromatic hydrocarbons and into a bottoms fraction containing aromatic hydrocarbons;

(c) introducing at least a part of said overhead fraction into an extraction means wherein said fraction is contacted by lean solvent under conditions sufficient to remove aromatic hydrocarbons;

(d) withdrawing from the extraction means a rafiinate stream having substantial freedom from aromatic hydrocarbons;

(e) withdrawing from the extraction means a rich solvent stream containing aromatic hydrocarbons;

(f) recovering a first aromatic product from said rich solvent stream; and

(g) separating said bottoms fraction into a second aromatic product stream and into a lean solvent stream having substantial freedom from aromatics.

References Cited UNITED STATES- PATENTS 2,337,669 12/ 1943 Larson 2083 18 2,344,406 3/ 1944 Hibshman 208-321 3,146,190 8/1964 Papadopoulos 208-313 3,209,047 9/ 1965 Young 208321 DELBERT E. GANTZ, Primary Examiner.

H. LEVINE, Assistant Examiner. 

